1. Field of the Invention
The present invention concerns a compact image sensor. This sensor can be applied notably in endoscopy. In this application, it is clearly necessary for the sensor to occupy as little space as possible while, at the same time, preserving acceptable electro-optical performance characteristics with regard to photosensitive surface, resolution, sensitivity, etc.
2. Description of the Prior Art
FIGS. 1 and 2 give a schematic view of a compact image sensor of the prior art. FIG. 1 shows a schematic cross-section of this sensor and FIG. 2 shows a schematic section along AA of the sensor of FIG. 1. The same references are repeated for the same elements in FIGS. 1 and 2.
This sensor has an imperviously sealed package 1, made of an electrically insulating material, for example a molded plastic material. This package has a front face 2, with a window 3, made of an electrically insulating material, which is transparent to light rays R coming from a part of an organ of the human body or from an object for which it is sought to obtain an image. The material forming the window 3 is made, for example, of glass or of a transparent plastic material. It is clear that when this sensor is used to obtain images of the inside of a human organ, the inside of this organ is illuminated by a light source which is independent of the sensor.
This sensor also has a rear face 4, forming the back of the package 1. This rear face is parallel to the front face.
Between the back 4 and the window 3, the inside of the package contains a stack arrangement comprising an insulating support 5, formed, for example, by a ceramic, and a set 6 of photosensitive elements on a semiconducting substrate 7, shown in a non-detailed way in the figure.
This substrate may be, for example, silicon while the photosensitive elements form a matrix consisting of photodiodes which may possibly be associated with charge-coupled devices or (CCDs).
As shown more precisely in FIG. 2, the photosensitive elements form a surface S bounded by a periphery P having contact elements or contacts such as 8. These contacts are connected to the photosensitive elements of the set 6, for example by connection tracks such as 9.
This sensor also has connection means used to connect the contacts 8 with connection terminals 10. These connection terminals go through the back 4 of the package 1. Herein, for each contact 8 and each corresponding terminal 10, there s a conducting strap 11 and an intermediate conducting part 12. The conducting strap 11 is metallic and flexible: it is made of copper for example. This strap is in contact with the contact element 8 and with the intermediate part 12, which itself is in contact with the terminal 10. This intermediate part too may be made of copper. Electrical conduction is thus set up between the terminal 10 and the contact 8. The various terminals of the sensor provide for the electrical supply to the photosensitive elements corresponding to the different contacts. The terminals 10 also enable the collection of the electrical signals given by the photosensitive elements when the photosensitive surface S is illuminated by light rays R. The electrical contacts between each contact element 8, strap 11, intermediate part 12 and terminal 10 are provided through the mechanical pressure that the package exerts, as it is being molded, on the window 3.
In this type of sensor, the area of the photosensitive surface S amounts to less than half of that of the front surface of the package 1, bounded by its external rim F. In a package with a front surface area of 16 mm.sup.2 for example, the photosensitive surface S occupies only 43% of this front surface. The remaining part is occupied, to a great extent, by all the connection elements (straps 11, intermediate parts 12) connecting the contacts 8 to the corresponding terminals 10. In the example considered, for a package with a front surface area of 16 mm.sup.2, the number of terminals 10 is restricted to six, and it is impossible to increase this number without increasing the front surface area of the sensor, precisely because of the excessive amount of space that the connection elements take up in the package. It is undesirable to increase the front surface area, especially for endoscopy. The result of this is that, in the prior art, there is no known sensor which, for the type of structure described, and for one and the same front surface area, has a photosensitive surface with a far greater area and with more connection terminals. This is a major drawback, because the performance characteristics of this sensor are limited.
The invention is aimed precisely at overcoming these drawbacks by making a compact image sensor wherein, without increasing the area of the front surface, it is possible to very substantially increase the area of the photosensitive surface as well as the number of connection terminals. These aims are achieved, as shall be seen further below in detail, by means of a judicious stacking arrangement within the package and through the use of far more compact connection elements.